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|Operating System Interaction|
The predicates in this section provide basic access to the operating
system that has been part of the Prolog legacy tradition. Note that more
advanced access to low-level OS features is provided by several
libraries from the
clib package, notably library
shell(Command, 0)’. See shell/2 for details.
On Windows, shell/[1,2]
executes the command using the CreateProcess() API and waits for the
command to terminate. If the command ends with a
& sign, the command is handed to the WinExec() API,
which does not wait for the new task to terminate. See also win_exec/2
Please note that the CreateProcess() API does not imply the
Windows command interpreter (cmd.exe and therefore commands that
are built in the command interpreter can only be activated using the
command interpreter. For example, a file can be copied using the command
?- shell('cmd.exe /C copy file1.txt file2.txt').
Note that many of the operations that can be achieved using the shell
built-in commands can easily be achieved using Prolog primitives. See
make_directory/1, delete_file/1, rename_file/2,
etc. The clib package provides
implementing various high level file operations such as copy_file/2.
Using Prolog primitives instead of shell commands improves the
portability of your program.
library(process) provides process_create/3
and several related primitives that support more fine-grained
interaction with processes, including I/O redirection and management of
time. For details, please consult the C library locale documentation. See also section 2.19.1. Please note that the locale is shared between all threads and thread-safe usage of setlocale/3 is in general not possible. Do locale operations before starting threads or thoroughly study threading aspects of locale support in your environment before using in multithreaded environments. Locale settings are used by format_time/3, collation_key/2 and locale_sort/2.
The predicates in this section are only available on the Windows version of SWI-Prolog. Their use is discouraged if there are portable alternatives. For example, win_exec/2 and win_shell/2 can often be replaced by the more portable shell/2 or the more powerful process_create/3.
SW_*constants written in lowercase without the
shownormal. In addition,
iconicis a synonym for
exploreor another operation registered with the shell for the given document type. On modern systems it is also possible to pass a URL as File, opening the URL in Windows default browser. This call interfaces to the Win32 API ShellExecute(). The Show argument determines the initial state of the opened window (if any). See win_exec/2 for defined values.
win_shell(Operation, File, normal).
DWORD, the value is returned as an integer. If the value is a string, it is returned as a Prolog atom. Other types are currently not supported. The default‘root' is
HKEY_CURRENT_USER. Other roots can be specified explicitly as
HKEY_USERS. The example below fetches the extension to use for Prolog files (see
README.TXTon the Windows version):
?- win_registry_get_value( 'HKEY_LOCAL_MACHINE/Software/SWI/Prolog', fileExtension, Ext). Ext = pl
CSIDL_and mapping the constant to lowercase. Check the Windows documentation for the function SHGetSpecialFolderPath() for a description of the defined constants. This example extracts the‘My Documents' folder:
?- win_folder(personal, MyDocuments). MyDocuments = 'C:/Documents and Settings/jan/My Documents'
%PATH%is extended with the provided directory. AbsDir may be specified in the Prolog canonical syntax. See prolog_to_os_filename/2. Note that use_foreign_library/1 passes an absolute path to the DLL if the destination DLL can be located from the specification using absolute_file_name/3.
is passed an absolute path to a DLL on a Windows installation
that supports AddDllDirectory() and friends,138Windows 7
with up-to-date patches or Windows 8. SWI-Prolog uses
LoadLibraryEx() with the flags
LOAD_LIBRARY_SEARCH_DEFAULT_DIRS. In this scenario,
%PATH% and not searched.
Additional directories can be added using win_add_dll_directory/2.
Representing time in a computer system is surprisingly complicated. There are a large number of time representations in use, and the correct choice depends on factors such as compactness, resolution and desired operations. Humans tend to think about time in hours, days, months, years or centuries. Physicists think about time in seconds. But, a month does not have a defined number of seconds. Even a day does not have a defined number of seconds as sometimes a leap-second is introduced to synchronise properly with our earth's rotation. At the same time, resolution demands a range from better than pico-seconds to millions of years. Finally, civilizations have a wide range of calendars. Although there exist libraries dealing with most of this complexity, our desire to keep Prolog clean and lean stops us from fully supporting these.
For human-oriented tasks, time can be broken into years, months, days, hours, minutes, seconds and a timezone. Physicists prefer to have time in an arithmetic type representing seconds or fraction thereof, so basic arithmetic deals with comparison and durations. An additional advantage of the physicist's approach is that it requires much less space. For these reasons, SWI-Prolog uses an arithmetic type as its prime time representation.
Many C libraries deal with time using fixed-point arithmetic, dealing with a large but finite time interval at constant resolution. In our opinion, using a floating point number is a more natural choice as we can use a natural unit and the interface does not need to be changed if a higher resolution is required in the future. Our unit of choice is the second as it is the scientific unit.139Using Julian days is a choice made by the Eclipse team. As conversion to dates is needed for a human readable notation of time and Julian days cannot deal naturally with leap seconds, we decided for the second as our unit. We have placed our origin at 1970-01-01T0:0:0Z for compatibility with the POSIX notion of time as well as with older time support provided by SWI-Prolog.
Where older versions of SWI-Prolog relied on the POSIX conversion functions, the current implementation uses libtai to realise conversion between time-stamps and calendar dates for a period of 10 million years.
We use the following time representations
. DST is
trueif daylight saving time applies to the current time,
falseif daylight saving time is relevant but not effective, and
if unknown or the timezone has no daylight saving time.
localto extract the local time,
’UTC'to extract a UTC time or an integer describing the seconds west of Greenwich.
?- date_time_stamp(date(2006,7,214,0,0,0,0,-,-), Stamp), stamp_date_time(Stamp, D, 0), date_time_value(date, D, Date). Date = date(2007, 1, 30)
When computing a time stamp from a local time specification, the UTC offset (arg 7), TZ (arg 8) and DST (arg 9) argument may be left unbound and are unified with the proper information. The example below, executed in Amsterdam, illustrates this behaviour. On the 25th of March at 01:00, DST does not apply. At 02.00, the clock is advanced by one hour and thus both 02:00 and 03:00 represent the same time stamp.
1 ?- date_time_stamp(date(2012,3,25,1,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -3600, TZ = 'CET', DST = false, Stamp = 1332633600.0. 2 ?- date_time_stamp(date(2012,3,25,2,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -7200, TZ = 'CEST', DST = true, Stamp = 1332637200.0. 3 ?- date_time_stamp(date(2012,3,25,3,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -7200, TZ = 'CEST', DST = true, Stamp = 1332637200.0.
Note that DST and offset calculation are based on the POSIX function
mktime(). If mktime() returns an error, a representation_error
dst is generated.
|Calendar year as an integer|
|Calendar month as an integer 1..12|
|Calendar day as an integer 1..31|
|Clock hour as an integer 0..23|
|Clock minute as an integer 0..59|
|Clock second as a float 0.0..60.0|
|Offset to UTC in seconds (positive is west)|
|Name of timezone; fails if unknown|
date(Y,M,D,H,M,S,O,TZ,DST)or a term
fcan be prefixed by an integer to print the desired number of digits. E.g.,
%3fprints milliseconds. This format is not covered by any standard, but available with different format specifiers in various incarnations of the strftime() function.
pmin lower case.
’%a, %d %b %Y %T %z'). Our implementation supports
%:z, which modifies the output to HH:mm as required by XML-Schema. Note that both notations are valid in ISO 8601. The sequence
%:zis compatible to the GNU date(1) command.
The table below gives some format strings for popular time
representations. RFC1123 is used by HTTP. The full implementation of
as available from
library(http/http_header) is here.
http_timestamp(Time, Atom) :- stamp_date_time(Time, Date, 'UTC'), format_time(atom(Atom), '%a, %d %b %Y %T GMT', Date, posix).
posix, which currently only modifies the behaviour of the
Bformat specifiers. The predicate is used to be able to emit POSIX locale week and month names for emitting standardised time-stamps such as RFC1123.
parse_time(Text, _Format, Stamp). See parse_time/3.
Date = date(Year,Month,Day). Days of the week are numbered from one to seven: Monday = 1, Tuesday = 2, ... , Sunday = 7.
The Windows executable swipl-win.exe console has a number of predicates to control the appearance of the console. Being totally non-portable, we do not advise using it for your own application, but use XPCE or another portable GUI platform instead. We give the predicates for reference here.
win_window_titlefor consistent naming.
notopmost. Topmost windows are displayed above all other windows.
true, show the window, if
falsehide the window.
selection_background. RGB is a term
rgb(Red,Green,Blue)where the components are values between 0 and 255. The defaults are established using the Windows API GetSysColor().
&is underlined and defines the associated accelerator key. Before is the label before which this one must be inserted. Using
adds the new entry at the end (right). For example, the call below adds an Application entry just before the Help menu.
inserts a separator. Goal is called if the user selects the item.